Magnetic sensors are increasingly important in various industries. For instance in the automotive industry various sensors such as parking sensors, angular sensors e.g. in throttle valves, ABS (Automatic Braking System) sensors and tire pressure sensors can be found in modern vehicles for improving comfort and safety. Magnetic sensors are particularly important in automotive applications, because magnetic fields penetrate easily through most materials. Magnetic sensors are also highly insensitive to dirt, unlike for example optical sensors.
Several different magnetic sensor technologies are currently available, such as magneto transistors (MT), sensors based on the Hall Effect and sensors based on the magnetoresistive effect such as anisotropic magnetoresistive (AMR) and giant magnetoresistive (GMR) sensors. The sensing principle of AMR sensors is based on the physical phenomenon that the electric resistance of a ferromagnetic material depends on the angle between the magnetization and the direction of the electric current within an AMR sensing element. Hall sensors and MTs, which rely on the Lorentz force, have a comparatively low sensitivity and consequently also a low accuracy. AMR sensors, while having a much higher sensitivity compared to Hall effect sensors and MTs, require more fabrication steps because they cannot easily be integrated monolithically, making a total sensor system ore expensive.
GMR sensors typically have a higher sensitivity than AMR sensors. However, a GMR sensor consists of various thin layers and critical interfaces. The technology required to fabricate such sensors in considerably more complicated and expensive. Furthermore, due to the thin multiple layers making up a GMR sensor, the operating temperature range is also limited. Therefore often AMR sensors are chosen as a good compromise in magnetic sensor applications.
U.S. Pat. No. 7,141,967 B2 discloses a magnetic field sensor system for measuring the angular speed of a soft magnetic (gear) wheel attached for instance to the axis of a camshaft or crankshaft in a transmission system of a combustion engine. The disclosed magnetic field sensor system comprises a MR sensor arrangement consisting of four MR sensor elements which are electrically connected with each other in a so called Wheatstone bridge. The magnetic field sensor system further comprises a hard magnetic (permanent) magnet that produces a biasing magnetic field which magnetizes the wheel. In this respect soft magnetic means that the wheel has a high magnetic permeability and hard magnetic means that the magnet can be permanently magnetized. The hard magnet is predominantly magnetized in a radial z-direction being oriented perpendicular to an x-y plane in which the four sensor elements are arranged. A periodic teeth-gap structure of the soft magnetic wheel produces periodic magnetic field changes when the wheel rotates, which are mainly visible in the y- and z-components of the magnetic field. Since the MR sensor arrangement is sensitive to the y-component (Hy) of the magnetic field (H), this periodicity is visible in the output of the magnetic field sensor system. This signal can be used for determining the position and speed of the (gear) wheel and therefore of the parts of e.g. an engine to which the magnetic (gear) wheel is mechanically connected. The known magnetic field sensor system has the disadvantage that external magnetic field may have a negative impact on the sensitivity and on the reliability of the magnetic field sensor system. Such a negative impact may in particular occur when the magnetic field sensor system is used for determining the position and/or the speed of a crankshaft sensor because the starter motor in a start-stop system is typically located in the neighborhood of the crankshaft.
There may be a need for providing an MR sensor concept which on the other hand has a high sensitivity and on the other hand exhibits a high immunity with respect to external magnetic fields.